The present invention generally relates to focus detecting devices for use in photographing apparatuses such as a camera, etc. and more particularly, to a focus detecting device having a function of detecting blur, which is provided with an operational system for detecting blur.
Conventionally, in photographing apparatuses such as a camera, etc., when blur is produced during taking of a photograph of an object to be photographed, such a phenomenon may take place that image quality of the photograph deteriorates and thus, it becomes impossible to obtain a satisfactory photograph. In order to prevent the undesirable phenomenon referred to above, cameras having a function of detecting blur have been proposed. Detection of blur is based on a principle that image signals of an identical object to be photographed are obtained twice at sequential points of time by an identical image forming system such that correlation between the two image signals is determined. If blur is not produced, the two image signals coincide with each other without displacing a spatial axis of one of the image signals. On the contrary, if blur is produced, a maximum correlation degree therebetween is obtained when a spatial axis of one of the image signals is displaced through a certain amount so as to coincide with that of the other one of the image signals. Since detection of blur is based on such principle, such a problem arises that in the case where the function of detecting blur is exercised, the object is required to be photographed twice in time series, thereby resulting in a time-consuming operation.
Meanwhile, in order to make cameras compact, several focus detecting devices combined with a function of detecting blur have been proposed. For example, U.S. Pat. No. 4,527,053 discloses a device for performing focus detection and detecting relative motion between an object to be photographed and a camera. A signal for detecting the relative motion is produced from a signal obtained from a focus detecting element such that the relative motion is detected upon completion of focus detection of the focus detecting element. Namely, in the device of this U.S. Patent, blur is detected by comparison of focus detecting signals generated at different points of time. Meanwhile, Japanese Patent Laid-Open Publication No. 259907/1985 discloses a blur detecting device in a distance detecting device of active (light emitting) type, in which distance detection is performed at least twice in time series such that amount of blur is determined. Furthermore, Japanese Patent Laid-Open Publication No. 259908/1985 discloses a device including a storage means for storing image signals obtained by a signal detecting element, in which the image signals stored at sequential points of time by the storage means are selected so as to be applied to a detection means for detecting amount of blur such that amount of blur is detected. However, all of these known devices are focus detecting devices combined with a function of detecting blur. Thus, the known devices conform to an object for taking a photograph of high image quality. However, in the known devices, focus adjustment is time-consuming due to a time period required for detecting blur and thus, it is impossible to repeat focus detection continuously and rapidly. Hence, in the known devices, in the case where focus adjustment is required to be performed continuously, for example, an photographer wishes to take a photograph rapidly at the time of a shutter chance and thus, such an inconvenience is incurred that the photographer may lose the shutter chance.
Photographic situations differ between a case where a photographer wishes to take a photograph of high image quality free from blur and a case where a photographer wishes to take a photograph on emergent occasions without losing a shutter chance. Therefore, it is generally desirable that focus detecting devices can be changed over to the different photographic situations. However, the above described known devices including a blur detecting mechanism have such a drawback that since the known devices are not provided with a changeover mechanism for changing over the known devices to the different photographic situations, it is impossible to take a photograph rapidly.